Spatial disorientation is a common feature of many organic brain disorders and psychiatric conditions. This investigation concerns how the limbic system and posterior parietal cortex (PPC) interact to form a stable spatial representation of the external world. The rodent anterior thalamus contains neurons called Head Direction (HD) cells that encode the direction the animal is facing in a familiar environment. This directional signal reflects the position of visual landmarks within an environment as well as internal self-motion signals. Theorists have proposed that the anterior thalamus receives either or both of these two types of information via PPC; thus, specific predictions regarding the effects of PPC lesions on the spatial representation of these cells may be made. This investigation will use a combination of lesioning and recording techniques to test these predictions. Following real or sham lesions of rat PPC, single cell activity in anterior thalamus will be recorded. If PPC is necessary for relaying landmark information to anterior thalamus, then the activity of HD cells following PPC lesions should no longer reflect the influence of landmark cues. Similarly, if PPC is necessary for relaying movement-related information to anterior thalamus, then the activity of HD cells following PPC lesions should no longer reflect this influence. Characterizing the nature of the interaction between PPC and the head direction system will significantly advance our understanding of the neurophysiological basis of spatial orientation.